The invention relates to particulate transport; more particularly it relates to particulate transport in oilfield stimulation treatments; even more particularly it relates to proppant and gravel transport slurried in viscous carrier fluids having insufficient viscosity to prevent particulate settling; even more particularly it relates to the inclusion in such fluids of fibers that aid in transport and inhibit settling; most particularly it relates to such fibers that degrade after the treatment into degradation products that do not precipitate in the presence of ions in the water such as calcium and magnesium, and to the selection of crosslinked polymer carrier fluids that are not damaged by contaminants present in the fibers or degradation products released by premature degradation of the fibers.
Hydrocarbons (oil, condensate, and gas) are typically produced from wells that are drilled into the formations containing them. Often, for a variety of reasons, such as inherently low permeability of the resource-containing formation or damage to the formation caused by drilling and completion of the well, the flow of hydrocarbons into the well and thus to the surface is undesirably low. When that is the case, the well is often “stimulated”. One of the most common forms of stimulation is hydraulic fracturing, in which a fluid is injected into the well and then into the formation at a pressure above the “fracture” pressure of the formation. A fracture is formed and grows into the formation, greatly increasing the surface area through which fluids may flow into the well. When the injection pressure is released, the fracture closes; consequently, a particulate material, called a “proppant” is included in the fracturing fluid so that when the pressure is released, the fracture cannot close completely, but rather closes on the proppant and the fracture faces are thus held apart by a bed of proppant through which the fluids may then flow to the well. The fracturing fluid normally must have a minimal viscosity that serves two purposes. First, the more viscous the fluid the more readily the fracture will be widened by injection of more fluid, and, second, a more viscous fluid will more readily transport proppant, hence the term “carrier” fluid. However, when the fluid is viscosified with a polymer, as is often the case, especially with a crosslinked polymer, at least some of the polymer or crosslinked polymer is left in the fracture after the treatment. This viscosifier left in the fracture inhibits the flow of desirable fluids out of the formation, through the fracture, into the wellbore, and to the surface for recovery. To some extent the need for viscosity can be offset by injecting fluid at faster rates, but for a variety of reasons, such as the limitations of the equipment and the costs, this is not always a desirable procedure. The viscosity may also be provided by non-polymeric methods, such as the use of foams, emulsions, and viscoelastic surfactant fluid systems, but sometimes these may not be the solution of choice. Operators may also choose to use the least damaging polymers available, but these may be expensive.
One solution some operators have chosen to minimize cost and polymer damage is to use as little polymer as possible. One such method is slickwater (also called waterfrac) treatments (with minimal proppant and a fluid viscosity, for example, of only about 3 cP, as opposed to conventional jobs with crosslinked polymer carrier fluids that typically have viscosities of at least 100 cP, and usually much more. To make up for the low viscosity, such jobs are usually pumped at high rates to help create the fracture and to transport the proppant, but fracture height growth may be excessive, very little proppant is placed, and the proppant may settle into the bottom of the fracture. This settling can occur as the fluid breaks or simply as a result of inadequate initial designed viscosity. Sometimes operators try to compromise with a combination of hybrid of a slickwater and a conventional job, which may result in the disadvantages of each.
Typically, when operators choose to use a more conventional fracturing method with polymer-based carrier fluids, they try to use the lowest possible polymer concentration, to minimize the damage caused by the polymer. It has recently been found that fibers included in the slurry of proppant in carrier fluid may serve to aid in the transport of proppant at lower viscosities and/or lower slurry flow rates (see SPE 68854 and SPE 91434) provided that fibers of the appropriate length, diameter, and stiffness are chosen and used in the right concentration. Such fibers also have the advantages that they improve the properties of the proppant pack, such as its fluid conductivity, its ability to aid in sand control, and resistance to flowing back of proppant particles into the wellbore. However, although the treatments have been very successful, there is still room for improvement; the materials previously typically used for the fibers either (glass or novoloid) did not degrade under formation conditions or did not have optimized stiffness for proppant transport or (polyethylene terephthalate) degraded into products that could reduce the final effectiveness of the fracture.